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Despite its bumpy landing, Philae -- the first human-made object to land on a comet -- managed to capture a cornucopia of chemistry data.

Eight months after that eventful touchdown, mission researchers are offering their initial interpretation of what the European Space Agency's probe found on comet 67P/Churyumov-Gerasimenko. The Philae results, published Thursday in the journal Science, include four molecules never before detected on a comet. But to experts, the most important thing is that the data confirm several expectations about comet composition.

Comet 67P/Churyumov-Gerasimenko on July 7, at a distance of 154 km from the Rosetta orbiter. Credit: ESA/Rosetta/NAVCAM

The question of what makes up a comet is actually pretty profound. Dust is a component, as are water ice and frozen gases such as carbon dioxide. But are they in forms Earth scientists would recognize? What else might be present? The answers are important for two reasons, according to Fred Goesmann, a Philae team leader based at the Max Planck Institute for Solar System Research in Göttingen, Germany. "Comets are considered to be very old, so they offer a window in time to the early ages of the solar system. Secondly, they probably formed far from the sun. Hence they provide a window in space to the areas far away from Earth." A third reason: Some scientists speculate that comets delivered materials to Earth that could have contributed to the origins of life.

On to the chemistry. Two of the Science papers focus on analyzing the comet's chemical makeup. While the two instruments featured, one in each paper, have different specialties, both machines are essentially scales for tiny stuff: they "weigh" molecules and molecule fragments. The mission's chemists identify molecules in their samples by finding matches for their data from prior research, or from a database.

The Rosetta spacecraft deposited Philae onto the 2.5-mile wide comet's surface in November. The idea was for Philae to latch on tight, drill down to the subsurface, and then analyze that sample's composition. That hasn't happened.

Scientists, though, are the types of people who come up with contingency plans. The data in Thursday's papers came from analyzing whatever dust, gases, and other material that happened to come Philae's way.

As a just-in-case measure, Philae was programmed to go into "sniff mode" 20 minutes after touchdown. "We opened up our instrument to the outside world to simply see what's in the air," says Ian Wright of the Open University in the U.K. Wright is the lead investigator for the shoebox-sized instrument called Ptolemy. Ptolemy's "sniffs" turned up polyoxymethylene, a large molecule made from repeating units of formaldehyde. That's not a big surprise. As early as the 1970s, researchers had surmised that interstellar grains may contain this molecule. Ptolemy also detected compounds scientists expect in a comet, like H2O and CO2.

That's not ho-hum information. Ptolemy could've sent back the data equivalent of gobbledygook, Wright says, which wouldn't have been out of the question given the landing snafu. But it didn't. It gave a signal consistent with what scientists already know or suspect.

While Ptolemy was sniffing away for Wright's team, Goesmann's team got a lucky break. Their Cometary Sampling and Composition (COSAC) instrument encountered a chunk of solid material kicked up by the lander's impact. Their analysis of that sample detected a mixture of 16 compounds. Four of them have never before been detected on a comet: methyl isocyanate, acetone, propionaldehyde, and acetamide.

Credit: Carmen Drahl

The teams made do under tough circumstances, and for that the work is "an incredible feat of analytical sciences," says John Plane, a member of the Atmospheric and Planetary Chemistry group at the U.K.'s University of Leeds who is not involved with the mission. However, the newly-discovered chemical foursome wasn't necessarily a surprise to him. He says lab experiments have shown that the compounds can form by mimicking conditions scientists expect to find on a comet-- by bombarding gases trapped in ice with cosmic rays and ultraviolet light, for example. That said, he adds, "it is exciting to see confirmation that these processes occur so actively in the solar system."

Sandra Pizzarello agrees. An emeritus professor at Arizona State University and an expert in the makeup of meteorites, Pizzarello points out that both papers' findings jibe with observations of similar chemical compounds in the interstellar medium-- the matter that occupies gaps between stars.

These two studies are not the last word on comet chemistry by any stretch. "This is all about doing the best we can do with what we've got, given the circumstances of the landing," Wright says. "This is our first attempt to put things into some kind of context," and the conclusions might not all stand up to scrutiny, he adds. The two chemistry teams had been working separately by design, but in the coming weeks, Wright says, they'll be meeting to hash out some of the similarities and differences in their data. (For instance, COSAC reported compounds containing nitrogen atoms while Ptolemy detected very low nitrogen compound levels.)

The teams have more data to analyze; Philae sent back "sniffs" until its solar-powered batteries ran down. The lander is awake again these days. But without a drilled sample from the comet surface, it's going to be hard for the lander's chemical instruments to strut their stuff. To add insult to injury, Philae isn't phoning home as reliably as the mission team would like. The connection "keeps dropping out like a bad mobile phone signal," Wright says. "It's incredibly frustrating."

Philae's chemistry results are a case study in the bedrock of science: observation, confirmation, and repetition. It may not sound as glamorous that way. But that doesn't make it any less awesome. These are the first chemical measurements taken on the surface of a comet. And Pizzarello puts it simply: "It is always exciting to see things happening for the first time."